The invention is directed to improvements in vehicle brake systems. An external force vehicle brake system is known from U.S. Pat. No. 3,667,813, the wheel brake cylinders of which can be acted upon with pressure from a pressure supply device for braking via a brake valve which is actuatable by means of a brake pedal. A failure of the pressure supply device results in the complete failure of the external force vehicle brake system.
In at least one brake line of this vehicle brake system, electrically controllable valves are installed to reduce undesirable drive wheel slip, for instance, when beginning to drive on a road surface with less grip. To reduce drive wheel slip, one of the valves provides a bypass from the pressure supply device around the brake valve to the particular wheel brake the wheel of which is skidding because of excessive drive torque. Another valve, which is disposed directly following the brake valve, simultaneously prevents the outflow of pressure medium through the non-actuated brake valve. A further valve, which is connected to the brake line between the last-named valve and the wheel brake is opened whenever the drive wheel slip has dropped sufficiently or has ended, so that the brake line is pressure-relieved to a supply container of the pressure supply device. The valve that can be opened toward the supply container has the disadvantage that if it should lock in its opened position, it renders the associated wheel brake ineffective.
From U.S. Pat. No. 4,034,566, it is known to associate a brake booster with a master cylinder which supplies at least one closed and therefore inherently safe brake circuit, the brake booster having a booster cylinder, a booster piston, a tappet that is displaceable by means of a brake pedal toward the booster piston, a brake valve actuatable by the relative movements of the tappet toward the booster piston, and a pressure supply device joined with the brake valve. By means of the brake pedal, via the tappet, the brake valve is actuated for feeding in boosting pressures into the booster cylinder. If boosting pressures are absent, then the booster piston can be displaced by means of the driver's foot, via the tappet in order to generate emergency braking pressures.